I’ve fixed the interface issue when talking to Ayano, sorry about that.
As for the bug where Hana becomes invisible, I’ve searched everywhere but can’t find the cause — I don’t know why it fails.
Regarding the black heart events, they trigger as follows:
Aiko: by going to her house after 7 p.m.
Aoi: by going to her shop when it’s open.
Kuro: by going to her shop when it’s open.
Those are the girls I assume you’ve already unlocked